We show an optically induced AC electrokinetic technique that rapidly and continuously accumulates colloids on an
electrode surface resulting in a crystalline-like monolayer aggregation. We demonstrate colloidal aggregation for
particles ranging from 100 nm to 3 μm. Electrothermal hydrodynamics produce a microfluidic vortex that carries
particles in suspension towards its center where they are trapped by low-frequency AC electrokinetic forces. We
characterize the rate of particle aggregation as a function of the applied AC voltage and hence characterize trapping
kinetics of this technique. We show that inter-particle distance varies with frequency and we explain this in the light of
available theory.